Mist Pass differences between Render Layer and Viewer
Passing it through the viewer like that causes it to be treated an output color instead of a data pass, so Blender starts gamma correcting it. When you view a pass directly, it is not corrected and you just see the raw data. You can verify this by checking values in a particular region on the raw pass, and comparing them to linear values shown on the viewer (not the ones labeled "CM"). They'll be the same.